1. Field of the Invention
The present invention relates to the field of imaging by means of ultrasonic examination. More specifically, the present invention relates to a sparse two-dimensional transducer array for forming ultrasonic images which has predefined foci in the elevational plane at certain positions by means of a compound lens.
2. Background Information
A variety of prior art apparatus have been used for forming two-dimensional images of a subject under examination using ultrasonic imaging systems. Typically, such systems employ ultrasonic probes in a variety of configurations in order to generate ultrasonic reference pulses or "bursts" into the subject under examination, and receive reflected ultrasound due to these ultrasonic bursts. A variety of probe designs has been used. For example, early systems employed fixed-focus transducers which employed either a lens or an acoustic mirror for focusing the ultrasonic bursts and receiving the ultrasound due to the burst within the subject under examination. Of course, such fixed-focus apparatus suffered from defects such as the ability to change focus to focus at a variety of depths, and the ability to generate a high-quality two-dimensional image (profile) of the subject under examination. Other transducer designs were created over time in order to address these problems.
Mechanical probe designs used a single or a plurality of transducer elements moved in a predetermined fashion by mechanical means (e.g., a motor) in order to accomplish scanning in a scanning plane. Single element transducers had a fixed focus at a single depth in the scanning plane. Another type of transducer used in the prior art is known as the annular array wherein a plurality of ring-shaped transducer elements is arranged on the probe in a concentric fashion. The signals to and from each ring are provided with different electronic time delays, so as to focus the beam at different depths. Annular arrays provide focusing both in the scanning plane and in the elevational or thickness plane of the image. Because of variances in motor speed, and the upper physical limitations of scanning in such a design, such mechanical probes (both single element and annular array) have several constraints. The chief constraints of mechanical probes is that beam agility is not available. This means that the beam cannot instantaneously move from one point to another in the scanning plane, and speed is limited by the motor design. Because of this limitation, multiple transmit zones in B-scan Imaging and simultaneous Duplex Doppler have never been achieved with mechanical probes. Also Color Flow Imaging has been implemented using these designs only with limited success.
A last type of transducer probe is known as the one-dimensional transducer array. Typically, this type of probe comprises a plurality of transducer elements arranged in a single-dimensional array wherein the array is used for scanning and electronic focusing, in order to form a two-dimensional profile of the subject under examination, rather than using annular arrays or single-element designs. This provides a higher-quality two-dimensional profile of a subject under examination. However, it does have its limitations. Some of these limitations will now be discussed.
FIG. 1 illustrates a transducer probe which comprises a single pre-focused receiving and transmitting surface 100 which is used for generating an ultrasonic burst and receiving echoes from all depths to a focus A, a predetermined distance from surface 100. For any emissive surface or aperture with a focus at a position A, whether the focus is achieved electronically or by means of a fixed-focus transmissive surface, path length errors increase at a different rate for depths other than A depending on the size of the aperture. For example, at a position as shown in FIG. 1, or any other position than the actual focal point A, there is a reduction in the quality of the reflection. This is because, unlike focal point A, the path lengths of distances from all other positions to different points in the aperture of the emitting/receiving surface vary. For example, the path lengths z.sub.0, z.sub.1 and z.sub.2 to depth B are all different from various points a.sub.0, a.sub.1 and a.sub.2 on surface 100. These varying path lengths, known as aperture path length error, result in image degradation. For example, when using an aperture having endpoints a.sub.1 for surface 100, the path length error is the difference between the distances z.sub.1 and z.sub.0. An aperture having the endpoints a.sub.2 on surface 100 will have a path length error=z.sub.2 -z.sub.0. Thus, the greater the aperture for any depth other than the predetermined focus, the greater the aperture path length error. The phase error due to the path length errors (in degrees) is then defined as ##EQU1##
Having a small aperture for any given focus minimizes aperture error, however, resolution is lost. Thus, there is a trade-off between aperture size to achieve optimum resolution and phase errors outside the focal point which cause degradation of an image and a reduced depth of focus.
This degradation caused by aperture error can be avoided by electronically adjusting the focus for every depth by changing the curvature of the transmissive surface for imaging a particular location. This is known as "dynamic focusing." This has the net effect of producing a larger zone in which the reflections received are all in focus. Such a surface can only be achieved by electronic means (e.g., delays in transmission of reference pulses for different regions of the surface, and delays in reception of ultrasound echoes due to the reference pulses). It is desirable to keep the number of readjustments of the focus to a minimum. During transmit mode, with every readjustment of focus, the frame rate decreases, because a new pulse has to be sent and received. In the receive mode, fewer readjustments require less complexity in the electronics. The less the aperture error outside the focal point, the fewer readjustments are needed to provide coverage over a given range of depths. Aperture error therefore, is something which needs to be contended with and compromises between frame rate and image quality need to be made.
Yet another error which may occur in certain transducers is known as element error. This type of error occurs in prior art one-dimensional arrays which focus the beam at various positions by means of electronic delays in emission of ultrasonic bursts from the elements in the array. Such an array is typically one-dimensional, for example, having n elements for scanning. Focusing of the beam is provided in the scanning plane only. An array which provides scanning is illustrated as 200 shown in FIG. 2a, and may form focus A by means of electronic delays in the activation of each of the transducer elements as illustrated in the cross-sectional view FIG. 2b. The wavefront, due to the delays in transmission, is shown as 202 in FIG. 2b. Of course, because the elements cannot be infinitesimally small, as in an ideal lens shown as 210 in FIG. 2b, a path length error known as "element error" occurs. The actual wavefront 202 caused by the delays is different from the ideal lens 210 as shown in FIG. 2b. Element error is the difference between the ideal transmissive surface of an element (represented by the ideal lens 210 focusing at depth A) and the wavefront achieved by the delays in the transmission of the bursts and reception of echoes at each of the transducer elements. In other words, it is the maximum difference of the path lengths across an element. For small size elements this error is small and the dominant error is the path length error across the aperture. For large size elements, path length errors over an element become more significant than aperture path length errors. For example, the element error for a particular transducer element 220 which is used for forming a signal due to the echo from focus A, there is an element path length error of the total of the distance between the ideal lens 210, and the actual wavefront 202 due to the electronic delays in the activation of the transducer elements. The differences between distances z.sub.2 and z.sub.3 are referred to as elevational path length error because an ideal lens has all path lengths (e.g., z.sub.2 =z.sub.3) equal The actual wavefront from one edge 223 of the element during transmit, and ideal lens at position 224 while focusing at depth A is thus referred to as an element delay error.
In addition to these element path length errors in the scanning plane, because of the inherent design of one-dimensional arrays, no focusing is generally provided in the "elevational plane" at all. However, the concepts of path length error across apertures and elements can be extended to an array with elements in the elevational plane. Similar problems occur on receive.
Yet another design which has been frequently used in prior art ultrasonic imaging systems is the annular array such as 300 illustrated in a front view in FIG. 3a. The annular array has a plurality of concentric ring-shaped transducer elements 301-304. The annular array has a pre-set mechanical focus at a fixed location from the face of the transducer such as C shown in the cross-sectional view of FIG. 3b. There are several advantages of such a design. First, elevational plane focusing of the same quality in the scanning plane is obtained because of the elements being shaped as rings instead of rectangles. Second, the focusing of the beam at depths other than the preset focus (e.g., depths A, B and D of FIG. 3b) is achieved with smaller element path length errors, at least in principle, by means of electronic delays as in one-dimensional arrays. However, although mechanical scanning capability is provided in such a design, (typically performed using a motor which mechanically rotates or oscillates the surface about a central axis) the limits of motor design reduces the utility of annular arrays. Due to the mechanical nature of the device, beam agility is also very limited.
A theoretical two-dimensional array would provide beam agility in the scanning plane and would also provide electronic focusing in the elevational plane. Such designs do not exist because a practical configuration employing 100 elements in the scanning plane, and 40 elements in the elevational plane would require a very large number of channels due to the very large number of elements (4000 separate transmit and receive channels). Even exploiting symmetrical connections in the elevational plane, the number of channels required for interconnection of these elements is still approximately 1500 to 2000 channels, which is well beyond current technology in transducer construction. Increasing the size of elements in the elevational plane may provide a partial solution to this problem, reducing the number of channels required, however, element errors for the different foci then become unacceptably large.
Thus, prior art transducers for ultrasonic imaging have fundamental shortcomings, in providing elevational plane electronic focusing.